Suppression of undesired electromagnetic emissions is a continuing design object because of the large and growing number of machines and appliances used which employ electronic circuitry which produces such noise. In the United States and other countries, government regulations strictly limit the emissions of commercial products. U.S. Pat. No. 4,243,890 to Miller et al is of some general interest in this respect in that it discusses control of electromagnetic signals.
It is widely understood that matching drive power to the load being driven reduces such undesirable emissions. Some custom chip manufacturers provide chips in which the drivers are specified to match the load of the intended use. Such selection must be designed into the chip and can not be changed later.
The great majority of microprocessors employed in commercial applications such as personal computers, printers, and other modern devices having controllers are entirely standard, such as, for example, the 386 microprocessors sold in large number by Intel Corp. and Advanced Micro Devices, Inc. These provide output drive signals of a single level of electrical potential which is high enough to be effective with most apparatus which might be driven. When such conventional microprocessor drives lesser loads than can be supported by the driver, the currents on the bus within the microprocessor and on the line to the load are larger than is absorbed by the load and therefore build up (sometimes termed "overshoot"). This produces reflected current and oscillations on these conductors (sometimes termed "ringing"), which emit electromagnetic noise. Similarly, where the drive signal is too low, undershoot and ringing occur to produce electromagnetic noise.